Refactor EhFrameHdr.

Previously, EhFrameHdr section computed addresses to which FDEs are
applied to. This is not an ideal design because EhFrameHdr does not
know much about FDEs unless EhFrame passes the information to EhFrameHdr.
It is what we did.

This patch simplifies the code by making EhFrame to compute the
values and pass the cooked information to EhFrameHdr. EhFrameHdr no
longer have to know about the details of FDEs such as FDE encodings.

llvm-svn: 270393